Tumors must be able to induce angiogenesis in order to develop. Based on this mechanism, vascular-targeted therapies have been investigated in a variety of tumor types, with mixed results in clinical trials. Given the potential risks associated with these therapies, it is desirable to determine the characteristics of tissues (and hence, patients) that are most likely to respond to current inhibitors and identify additional therapeutic strategies. Angiogenesis occurs in a complex environment where endothelial cells are exposed to a variety of factors that are known to regulate angiogenesis, such as mechanical stiffness, extracellular matrix (ECM) density, and soluble growth factors. We hypothesize that the physical properties of the tumor microenvironment (e.g., stiffness, ECM density) impact tumor sensitivity to pro-angiogenic molecules, and therefore tumor responsiveness to vascular-targeted therapies. Aim 1: Evaluate how microenvironment properties impact endothelial cell (EC) responsiveness to soluble angiogenic stimuli. We will characterize the tumor microenvironment in a mouse model of breast cancer and apply this information to design a novel microfluidic-based culture system that enables independent variation of matrix stiffness and density. This system will be used to assess how different combinations of these characteristics impact cellular sensitivity to complex combinations of soluble angiogenic stimuli. We will then utilize computational modeling to analyze our experimental results in order to determine which angiogenic factors most strongly induce angiogenesis in the different physical microenvironments. Aim 2: Utilize tumor microenvironment properties and soluble factor combinations to predict EC responsiveness to vascular-targeted therapies. Using both our Aim 1 in vitro model and an in vivo mouse model of breast cancer, we will investigate whether the experimental and computational results gained in Aim 1 can be used to inform the selection of an optimal vascular-targeted strategies for a set microenvironment. By examining how the microenvironment regulates cellular sensitivity to angiogenic stimuli, this work aims to provide a foundation for predicting tumor responsiveness to vascular-targeted agents. The results obtained from these studies have the potential to inform the identification of treatment options for breast cancer.